Electron beam control system



Sept. 15, 1959 c. F. AULT 2,904,721

ELECTRON BEAM CONTROL SYSTEM Filed Dec. 31, 1956 2 Sheets-Sheet 2CONTROL I II N I @vERuoo/e GATE Q com: cars. 64 I INVENTOR C. E A UL TATTORNEY United States Patent Oflflce 2,904,721 Patented Sept. 15, 1959ELECTRON BEAM, CONTROL SYSTEM Cyrusv F. Ault, Ciifton, N.J., assignortov Bell Telephone Laboratories, Incorporated, New York, N.Y., acorporation of New York Application December 31, 1956, Serial No.631,554 21 Claims. (Cl. 315-85) This invention relates to electrondischarge apparatus and more particularly to cathode raydevicescomprising a plurality of target elements.

For various beam positioning, applications, notably in informationstorage systems, it. is essential to obtain rapid and accuratepositioning of the cathode ray tubebeam on the precise area of thestorage surface. from which. information is to be derived or at whichinformation is to. be stored.

The positioning system which will comply with these exactingrequirements and maintain a high performance standard over long periodsmust necessarily comprise a minimum of active elements subject to wearand consequent variation.

In order to appreciate more fully the requirements of such a positioningsystem, a storage system utilizing a barrier grid storage tube, asdescribed in R. W; Sears Patent 2,675,499, issued April 13, 1954,provides for upwards of 15,000 discrete information bearing areas on arelatively small storage surface, any one of which areas is to beselected by coordinate deflection of an electron beam in a matter ofmicroseconds and the information located thereat read out.

Initial positioning of the beam to achieve the precise coordinatedeflections required. in such a. system places a severe strain onpositioning equipment involving the conventional digital to analogconversion and direct current amplification of input signals to thedeflection plates, as known in the art.

Other approaches to the accurate positioning problem involve the use ofa positioning control device to monitor the storage tube beamdeflection. The control device may be a, cathode ray tube with a binarycoding mask as described in an application Serial No- 581,073, filedApril 27, 1956, of C. W. Hoover and R. W. Ketchl'edge, now Patent2,855,540, issued October 7., 1958". In. this instance the control tubebeam is deflected to a position corresponding to the desired position inthe storage tube. Output signals obtained frompassage of the controltube beam through the coding mask are compared with the input deflectionsignals and the resultant signals are applied to the deflection platesto reposition the beam to the precise desired position. The storage tubehas its delflection plates connected in parallel with those of thecontrol tube, so that the storage tube beam deflection matches that ofthe control tube.

In this example, as inother monitoring systems operating on a servopositioning, principle, the. feedback loop operation controls the beampositioning speed, and. the speed obtained thereby may not besuflicient, to satisfy requirements in high speed storage applications;

Accordingly, it is an object ofv thisv invention to provide an improvedbeam positioning system, and more specifically a beam positionng systemwhich is independent of voltage variations due toinstability and agingof active components.

It is another object of this invention to achieve accuratelyreproducible electron beam deflections in one or more cathode raydevices.

It is a further object of. this invention to facilitate the realizationof such accurately reproducible beam deflections.

It is a still further object of this invention to improve the beampositioning speed while maintaining requisite accuracy.

In one illustrative embodiment of this invention, a cathode ray devicecomprises an electron gun, means focusing the stream of. electrons fromthe gun on a target area, pairs of. overlapping collector platespositioned in cartesian coordinates of the target area, a deflectionsystem to deflect the beam to the collector plates and means foradjusting input signals to the deflection system in accordance withoutput signals from coordinate pairs of said collector plates. Thedeflection system of this device is connected in parallel toone or morestorage tube deflection systems to provide comparable beam deflectionstherein.

In one. construction the edges exposed to the electron beam formed byeach pair of overlapping collector electrodes. establish geometricpositions in space equidistant in their respective coordinates from thecoordinate intersection and on opposite sides thereof. Periodicallyinput voltage of. one polarity is applied to the deflection plates todrive the electron beam to one edge, and then an equal but oppositepolarity voltage is applied to the deflectionplates. to drive the beamto the other edge, intersecting thesame coordinate axis. The beamimpinging uniformly about each edge in one coordinate indicates accuratebeam positioning in that coordinate.

Should a majority of the vertically deflected beam impinge the collectorplate above each edge, intersecting the vertical coordinate, forexample, an upward drift error in the electron beam deflection circuitryis indicated, and a current flows from the upper collector plate in eachpair of vertical collector plates, serving to charge associatedcapacitors equally but in opposing polarity. The difference in voltagebetween the capacitors is detected and serves to alter the beamdeflection so as to correct for. the drift error. Similarly, drift inthe opposite direc- 'tion results in a difference voltage signal fromthe capacitors, of opposite polarity tending to compensate for drift inthe opposite direction.

Should the vertically deflected beam strike above the overlapping edgeof the upper pair of collector plates and below the overlapping edge ofthe lower pair of collector plates, or vice versa, an error in gain isindicated. The capacitors will be charged equally and with the samepolarity in this instance. Since no voltage difference exists betweenthe capacitors, no drift correction results. The voltage on eachcapacitor serves instead to 'vary the gain of the associated deflectioncircuitry to restore the proper beam deflection.

Thus, successive positioning of the monitor tube beam on two remotegeometric positions in one coordinate will permit repositioning of thebeam to compensate for errors in drift and gain derived from thedeflection system input circuitry for that coordinate.

In accordance with one feature of this invention, an electron dischargedevice comprises a plurality of pairs of overlapping collector platespositioned such that edges formed thereby intersect cartesiancoordinates in spaced apart positions.

Another feature of this invention relates to coordinate deflection of anelectron beam to impinge first and second pairs'of collector plates insuccession.

It is another feature of this invention that capacitors are charged inaccordance with the position of beam impingement' on the collectorplates and serve to formulate signal'sztoadjust for errorsin the beam.deflection.

A complete understanding of this invention and of the above-noted andother features thereof may be gained from consideration of the followingdetailed description and the accompanying drawing in which:

Fig. l is a diagrammatic representation of a cathode ray device andassociated circuitry illustrative of one embodiment of this invention;

Fig. 2 is a schematic representation of one specific embodiment of thisinvention in accordance with Fig. l; and

Fig. 3 is a diagram illustrating one manner in which devices as shown inFig. 1 may be utilized to produce coordinate deflecting potentials for aplurality of cathode ray devices.

Referring now to the drawing, Fig. 1 depicts a cathode ray tube andrelated circuitry in accordance with one specific illustrativeembodiment of this invention. The cathode ray tube, shown generally at10, comprises an evacuated envelope, such as of glass, an electron gun11, deflection plates 12 and 13, and overlapping pairs of collectorelectrodes 14 and 15, 16 and 17, 18 and 19, and 20 and 21, mounted inspace quadrature. Thus collector pairs 14, 15 and 16, 17 are positionedon the vertical axis equidistant from the intersection with thehorizontal axis and on opposite sides thereof. Similarly, collectorpairs 18, 19 and 21), 21 are positioned on the horizontal axisequidistant from the intersection with the vertical axis and on oppositesides thereof. Each of the vertical and horizontal axes is defined by abeam scanned across the tube face under control of one coordinatedeflection system and with zero signal on the deflection system for theother coordinate. Each pair of collectors overlaps to form a distinctedge exposed to the electron beam and advantageously intersecting therespective axes at right angles.

Coordinate deflection of the electron beam is performed in a manner wellknown in the art, in which input information is applied in parallelbinary form to input registers for each deflection coordinate. flectioncircuitry is identical to the circuitry for vertical deflection so thata description of the horizontal circuitry will suffice to describe thestructure and operation of this specific embodiment of this invention.

Defects in deflection circuitry result in errors in beam positioningclassified generally as drift errors and gain errors. A drift error isdue to a variation in potential in the same direction on both deflectionplates for one coordinate. Thus, a positive variation in potential onthe horizontal deflection plates will cause the beam to be deflectedconsistently above the desired deflection positions. A gain error is dueto a variation in potential in opposite directions on the deflectionplates for one coordinate. Thus, an increase in potential applied to theThe horizontal dehorizontal deflection plates, tending to increase thepoten- 1 tial difference therebetween, will cause upward beamdeflections above the desired deflection positions and downward beamdeflections below the desired deflection positions.

Positioning control devices known in the art, such as that described inthe patent of Hoover and Ketchledge cited hereinbefore, monitor allpossible beam positions, thereby providing an extremely accurate controlsystem. The monitored devices, however, are delayed in performing theirpeculiar functions after each application of input information while thebeam position is checked by the monitor device. Such delay cannot betolerated in many high speed memory operations.

Beam positioning errors occur gradually so that information contained ina specific storage tube may be scanned several times before errors occurof sufficient magnitude to produce spurious output information. Thus, inaccordance with this invention, beam repositioning is reduced toperiodic coordinate drift and gain correction. The circuitry andoperations involved in horizontal and vertical deflection correction areidentical, so

- 4 that a description of vertical deflection correction will suflice.

Binary information for vertical deflection is fed into input register25, converted to analog form in analog converter 26, amplified indeflection amplifier 27, and applied as opposite potentials todeflection plates 12. The input register 25 and its associated analogconverter 26 may be of any of a number of circuits capable of generatinganalog representations on simultaneous application thereto of aplurality of horizontal address input pulses; for example, as best shownin Fig. 2, input register 25 may comprise a series of bistable flip-flopunits 50 arranged to feed simultaneously through diodes 51 of analogconverter 26, which is capable of passing analog stepped amounts ofcurrent to deflection amplifier 27. Amplifier 27 advantageouslycomprises triodes 52 and 53 which supply output voltages to thedeflection plates 12 of opposite polarities representing a summation ofanalog values.

The vertical coordinate repositioning operation involves applyinghorizontal address input information to the deflection plates 12 withthe plates 13 at zero potential. If the horizontal address inputinformation is correct, the beam will be deflected in succession to theedges formed by the collector pairs 14, 15 and 16, 17.

In accordance with this embodiment of this invention, deflection of theelectron beam so as to straddle any of the edges formed by theoverlapping collector electrodes will produce equal currents in leads 30and 31 to differential amplifier 32. Advantageously, the odd numberedinner collectors 15, 17, 19, and 21 are commonly connected to lead 30and even numbered outer collectors 14, 16, 18, and 20 are commonlyconnected to lead 31. Thus the beam impinging any of the collectorplates will produce current flow in at least one of the leads 30 and 31to differential amplifier 32.

The output of differential amplifier 32 is received in capacitors 33 and34 after being directed through gates 35 and 36in accordance withsignals from gate control 64. If the beam is positioned so as to impingea pair of overlapping collector electrodes uniformly about the edge,there is no output from differential amplifier 32 and the capacitors 33and 34 are not affected.

If a positioning error is present which causes an upward or downwarddrift, the predominant portion of the beam will impinge upon thecollector electrode above or below each edge intersecting the verticaldeflection path, respectively, thereby generating more current flow inone of the leads 30 and 31. Differential amplifier 32 in turn willproduce output signals of opposite polarity which are directed throughgates 35 and 36 so as to increase the charge in opposite directions oncapacitors 33 and 34.

If a positioning error is present due to a variation in gain in thevertical deflection circuitry, the predominant portion of the beam willimpinge upon the collector electrode below one edge and the collectorelectrode above the other edge intersecting the vertical deflectionpath. At the former position the unbalanced current flow in leads 30 and31 will cause amplifier 32 to produce an output signal of one polarity,serving to charge one of the capacitors 33 and 34 in one direction. Atthe latter position the same unbalanced current flow in leads 30 and 31will cause amplifier 32 to produce an output signal which is gated so asto charge the other capacitor in the same direction.

Increases in charge on capacitors 33 and 34 in the same direction willgenerate a signal through adder '72, differential amplifier 38 andanalog converter 26 to the deflection plates 12, indicating an error ingain. Increases in charge in opposite directions on capacitors 33 and 34will produce an output signal from differential amplifier 39 to thedeflection plates 12, indicating a drift error. The various circuits inthis feedback path will be described in detail hereinafter.

Gates 35' and36, as best shown in- Fig. 2, advantageously comprise solidstate and vacuumtube diodes combined in parallel'push pull arrangementsnormally biased soas-to be nonconducting; Gate 36 contains two branchgates 62. and 63; Gate cont roli64 applies signals-eflective. toenablegate .35 and=branch-gate v62 in unison when the beam ispositionedonncollector plates 14; 15.. Similarly, with the. beampositioned-on collector. plates 16, 17, gate. control 64 applies signalsto enable gate 35 and branchgate 63 in unison. In this fashion only thecollector current signals produced. after the deflected. electron beamhascome .to..rest. on .a. pair of collector electrodes will be passed bygates 35' and-36;

Signals. passed: concurrently. by gate 35 and! branch gate. 62 chargecapacitor: 33zwhi'le :signals passed by gate 35: and. branch gate 63chargecapacitorrlid; The charge stored on capacitors 33 and.3.4;determines: the amount of error due, to drift and gain. Timingtheenablementof gates 35 and 36 assures, that the charges Qn capacitors33 and 34 will remain constant. while, the beam is being deflectedbetween collector electrodes 14, 15 and collectorelectrodes 16, 17.

Differential amplifier 38 is responsive to the charge on capacitors 33and 34 to provide output signals compensating'for errors due to gain.The. charge on capacitor 33 determines the grid potential of tube 70.Likewise, the charge on capacitor S I-determines the; grid-potential oftube 71, and the combined charges on capacitors 33 and 34 determine thegrid potential of adder tube 72. Voltage variations in'thecathodecircuitsof tubes 76 and 71 control the grid potential of tube 84in differential amplifier 33 While a voltage variation in the cathodecircuit of adder tube 72 varies the grid. potential of tube 83 therein.Output signals from differential amplifier 38-are fed over lead 86 tothe analog converter 26.

If a gain error is present, capacitors 33 and 34 will bev charged in thesame direction as described hereinbefore. Sufiicient delay is. builtinto the circuit to assure that both capacitors are charged according topositioning of the beam on. each. pair of overlapping collectorelectrodes prior to providing an output from the error compensatingcircuitry. Thus the cathode voltages of tubes 70 and 71 are varied inthe same direction and add so as to vary the grid potential of tube 84.Also, the cumulative charges on capacitors 33 and 34 alter the bias onadder tube 72 which in turn varies the grid po tential of tube 83opposite to that of tube 84. The output on lead 86 represents thedifference in potentials applied to the grids of tubes 83 and 84 and isapplied to the analog converter 26 to vary the gain of the deflectioncircuitry.

If a drift error is present, capacitors 33 and 34- will be charged inopposite directions. Thus the grid potential of adder tube 72 andconsequently tube 83 remains unchanged. Also resultant opposite voltagevariations in the cathode circuits of tubes 70 and 7.1 eflectivelycancel each other at the grid of tube 84 leaving its potential unchangedand no output from gain control diiferential amplifier 33 is provided.The oppositely directed Voltage variations in the cathode circuits oftubes 70 and 71 also are applied to the grids of tubes 75 and 76 indrift control diiferential amplifier 39 over leads 73 and 74,respectively, and are effective to produce an output therefrom asoppositely directed voltage variations in leads 80 and 81 to deflectionamplifier 27. Such output signals correct for the variation in beamdeflection due to drift which was responsible for the storage ofoppositely directed charges on capacitors 33 and 34. Capacitors 33 and34, when charged in the same direction due to a gain error, will serveto raise the grid potential of tubes 75 and 76 in like amounts, so thatno output from drift control differential amplifier will be provided atvthis time.

Drift and gain errors in the horizontal coordinate also are corrected inthe manner outlined above. Proper timing of the gate control64assurescharging of capacitor 33 with thebeam positioned'on one pair ofcollector electrodes in one coordinate and charging of capacitor 34 withthe beam positioned on the opposite pair of collector electrodes in thesame coordinate. The magnitude of charge on each capacitor 33 and 34will determine the magnitude of gain or drift correction applied to therespective deflection plates.

The primary functionof this arrangement, in accordance with thisembodiment of this invention, is to monitor the positioning of electronbeams in one or more storage or related tubes having their deflectionplates connected in parallel with the deflection plates of tube 10. Asshown in Fig. 3, for example, the deflection plates of monitor tube 10may be connected'to devices deflected in two coordinates such as tube 40or single coordinate deflection tubes such as 41 and 42. The monitoreddevices 40, 41, and 42 may be any cathode ray devices, but thepositioning arrangement of this invention is particularly useful inaccurate and. rapidpositioning of tubes of the storage type such as theBarrier Grid tube. The deflection plates 12 are-connected in parallelwith the deflection plates 43 of device 40 and deflection plates 45 ofdevice'41; the plates 13 similarly are connected to plates 44 of deviceand plates 46 of device 42. Hence the deflecting potential across anypair of deflection plates 43 and 45 will be the same as that across theplates 12, and that across plates 44 and 46 the same as that acrossplates 13. With the voltages across the respective deflection plates 12and 13' accurately representative of signal voltages impressed onmonitortube 10 when the beam is deflected to the collector electrode edgesinone coordinate, accurately reproducible deflections of the beams indevices such as 44), 41, and 42 may be obtained.

It is to be understood'that the above-described arrangements areillustrative of'the application of the principles of the invention.Numerous other arrangements may be devisedby those skilled in the artwithout departing from the spirit and'scope of the; invention.

What is claimed is:

1. A beam positioning system comprising an electron discharge deviceincluding target means and means for projecting a beam toward saidtarget means, storage means, means for gating a signal to said storagemeans responsive to said beam impinging said target means, meansdeflecting said beam.to impinge said target means successively in atleast two discrete deflected positions to produce at least two discretesignals, and means connected between said storage means and saiddeflection means transmitting deflection correcting indicationsv to saiddeflection means after receipt in said storage means of both of saidsignals.

2. A beam positioning system comprising an electron discharge deviceincluding a plurality of target elements, means for projecting a beaminitiated by a source in an electron discharge device toward said targetelements and deflection means, means for applying input signals to saiddeflection means. to position said. beam on said target elements, gatingmeans, storage means receiving a first signal through saidgating meansupon positioning of said beam on certain of said targetelements, saidstorage means receiving a second signal through said gatin means uponpositioning ofsaid beam on other of said target elements, meanscomparing said first andsecond signals, andmeans applying signalsresulting from said comparison to said deflection means.

3. A beam positioning system-: comprising an electron discharge deviceincluding target means and means for projecting a beam toward saidtarget means, means for deflecting said beam to impinge said targetmeans successively in at least two discrete positions, storage means,gating means connected between said storage means and said target meansapplying a first signal to said storage means. responsive to saidbeamimpinging said target means in a first discrete position, andapplying a. second '7 signal to said storage means responsive to saidbeam impinging said target means in a second discrete position, meansfor comparing said first and second signals, and means applying theresultant of said comparison to sald deflection means.

4. A beam positioning system in accordance with claim 3 wherein saidtarget means comprises a plurality of pairs of overlapping collectorelectrodes, a first pair of said overlapping collector electrodespresenting a first exposed edge to said beam intersecting a beamdeflection path and a second pair of said overlapping collectorelectrodes presenting a second exposed edge to said beam spaced apartfrom said first edge.

5. A beam positioning system in accordance with claim 4 wherein saidfirst and second edges are mutually parallel and intersect said beamdeflection path equidistant from the undeflected beam position in saidbeam deflection path.

6. A beam positioning system in accordance with claim 5 wherein saidstorage means comprises first and second capacitors.

7. A beam positioning system in accordance with claim 6 wherein saidsignal applying means comprises first and second gating means, saidfirst gating means applying said first signal to said first capacitorand said second gating means applying said second signal to said secondcapacitor.

8. A beam positioning system in accordance with claim 7 and furthercomprising gate control means connected to said first and second gatingmeans, said gate control means enabling said first gating means withsaid beam positioned on said first pair of collector electrodes andenabling said second gating means with said beam positioned on saidsecond pair of collector electrodes.

9. A beam positioning system in accordance with claim 8 wherein saidcomparison means comprises first and second comparison circuits, saidfirst comparison circuit providing an output responsive to said firstand second capacitors charged in the same direction, and said secondcomparison circuit providing an output responsive to said first andsecond capacitors charged in opposite directions.

10. A beam positioning system comprising an electron discharge deviceincluding means for projecting an electron stream substantially alongthe axis of said device, a pair of first and second collector electrodespositioned on each side of a plane through said axis, said firstelectrode in each pair being positioned closer to said axis than saidsecond electrode in each pair, means for causing said electron stream toscan successively one pair of said electrodes and then the other pair ofsaid collector electrodes, and means for applying correction signals tosaid scanning means when the predominant portion of said electron streamimpinges on said first electrode of one pair and said second electrodeof the other pair.

11. A beam positioning system comprising an electron discharge deviceincluding means for projecting an electron stream substantially alongthe axis of said device, a first set of collector electrodes on oppositesides of a plane through said axis, a second set of collector electrodeson opposite sides of said plane and positioned closer to said axis thansaid first set, means for causing said electron stream to scansuccessively said electrodes on one side of said plane and then saidelectrodes on the opposite side of said plane, and means for applyingcorrection signals to said scanning means when the pre dominant portionof said electron stream impinges on the same set of said collectorelectrodes.

12. A beam positioning system comprising an electron discharge deviceincluding means for projecting an electron stream substantially alongthe axis of said device, a set of first collector electrodes spacedapart from each other on opposite sides of a plane through said axis andsubstantially in a plane normal to said plane through said axis, a setof second collector electrodes difierently spaced from said axis thansaid first collector electrodes, means for causing said electron streamto scan successively a pair of said electrodes on one side of said planeand then a pair of said electrodes on the opposite side of said plane,means for applying correction signals to said scanning means when thepredominant portion of said electron stream impinges on said firstelectrode of one pair and said second electrode of the other pair, andmeans for applying different correction signals to said scanning meanswhen the predominant portion of said electron stream impinges on thesame set of said collector electrodes.

13. A beam positioning system comprising an electron discharge deviceincluding first and second target means, means opposite said targetmeans for projecting a beam toward said target means, and meansdeflecting said beam to impinge said first target means and said secondtarget means successively, storage means, gating means passing signalsto charge said storage means in response to said beam impinging saidfirst and second target means, and means providing an output to saiddeflection means indicative of the level of charge in said storagemeans.

14. A beam positioning system comprising an electron discharge deviceincluding at least two pairs of overlapping collector electrodes, meansopposite said electrodes for forming and projecting an electron beamtoward said electrodes, and means for deflecting said beam to strikesuccessively first and second pairs of said collector electrodes, firstand second storage means, means connected between said storage means andsaid electrodes applying a signal to said first storage means uponimpingement of said beam on said first pair of collector electrodes andapplying a signal to said second storage means upon impingement of saidbeam on said second pair of collector electrodes, means for comparingsaid signals stored in said first and second storage means, and meansfor applying the resultant of said comparison to said deflection means.

15. A beam positioning system in accordance with claim 14 wherein saidcollector electrodes are positioned in planes normal to the path of saidbeam when undefiected, said first and second pairs of overlappingcollector electrodes each presenting an exposed edge to said beam, saidedges being spaced apart and intersecting a deflection coordinate ofsaid beam.

16. A beam positioning system in accordance with claim 15 and furthercomprising third and fourth pairs of overlapping collector electrodeseach of said pairs presenting an exposed edge to said beam, said edgesbeing spaced apart and intersecting another deflection coordinate ofsaid beam.

17. A beam positioning system in accordance with claim 16 wherein saidsignal applying means comprises first, second and third gating means,said first storage means connected between said first and second gatingmeans and said second storage means connected between said first andthird gating means.

18. A beam positioning system in accordance with claim 17 wherein saidsignal applying means comprises differentiating means connected betweensaid gating means and said collector electrodes, said differentiatingmeans responsive to current flow from a pair of said overlappingcollector electrodes with said electron beam impinging thereat otherthan uniformly about the edge formed thereby to transmit a signal tosaid gating means.

19. A beam positioning system in accordance with claim 18 wherein saidsignal applying means comprises gate control means connected to saidfirst, second and third gating means, said gate control meanstransmitting enabling pulses to said gating means in accordance with thepositioning of said electron beam whereby said first and second gatingmeans are enabled with said beam positioned on said first pair ofcollector electrodes and said first and third gating means are enabledwith said beam positioned on said second pair of collector electrodes.

20. A beam positioning system in accordance with claim 19 wherein saidsignal comparing means comprises gain indication means and driftindication means, said gain indication means being connected to saidstorage means so as to provide an output responsive to signals stored inone direction in said storage means, and said drift indication meansbeing connected to said storage means so as to provide an outputresponsive to signals stored in opposite directions in said storagemeans.

21. A beam positioning system in accordance with claim 20 and furthercomprising means for applying deflection signals to said deflectionmeans, said gain indication means being connected to said deflectionsignal applying means and said drift indication means being connected tosaid deflection means.

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